EP0881304B1 - Verfahren und vorrichtung zur vakuum-entkohlung/feinung von flüssigem stahl - Google Patents

Verfahren und vorrichtung zur vakuum-entkohlung/feinung von flüssigem stahl Download PDF

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Publication number
EP0881304B1
EP0881304B1 EP97913417A EP97913417A EP0881304B1 EP 0881304 B1 EP0881304 B1 EP 0881304B1 EP 97913417 A EP97913417 A EP 97913417A EP 97913417 A EP97913417 A EP 97913417A EP 0881304 B1 EP0881304 B1 EP 0881304B1
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Prior art keywords
molten steel
vacuum
oxygen
snorkel
decarburization
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EP97913417A
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English (en)
French (fr)
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EP0881304A1 (de
EP0881304A4 (de
Inventor
Kenichiro Nippon Steel Corporation MIYAMOTO
Katsuhiko Nippon Steel Corporation Kato
Akio Nippon Steel Corporation SHINKAI
Takayuki Nippon Steel Corporation KANEYASU
Shinya Nippon Steel Corporation KITAMURA
Hiroyuki Nippon Steel Corporation ISHIMATSU
Hiroshi Nippon Steel Corporation SUGANO
Keiichi Nippon Steel Corporation KATAHIRA
Ryuzou Nippon Steel Corporation HAYAKAWA
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Nippon Steel Corp
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Nippon Steel Corp
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Priority claimed from JP32617896A external-priority patent/JPH10152721A/ja
Priority claimed from JP33756596A external-priority patent/JP3749582B2/ja
Priority claimed from JP34244296A external-priority patent/JP3754154B2/ja
Priority claimed from JP9120302A external-priority patent/JPH10298635A/ja
Priority claimed from JP9123186A external-priority patent/JPH10298634A/ja
Priority claimed from JP13429997A external-priority patent/JPH10310818A/ja
Priority claimed from JP22064097A external-priority patent/JP3785257B2/ja
Application filed by Nippon Steel Corp filed Critical Nippon Steel Corp
Publication of EP0881304A1 publication Critical patent/EP0881304A1/de
Publication of EP0881304A4 publication Critical patent/EP0881304A4/de
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/10—Handling in a vacuum
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/04—Removing impurities by adding a treating agent
    • C21C7/068—Decarburising

Definitions

  • the present invention relates to a method and apparatus for vacuum decarburization refining a molten steel and, more particularly, to a method and apparatus, for refining a molten steel, that can inhibit the deposition of a splash onto the inner wall of a vacuum tank and an oxygen lance and at the same time can prevent oxidation loss of metal in the molten steel.
  • Conventional methods for additional decarburization refining of a molten steel which has been once subjected to decarburization refining in an electric furnace or a converter to provide a molten steel having a carbon concentration of not more than 0.01% by weight include: (1) a VOD (vacuum oxygen decarburization) method, typified by the one disclosed in Japanese Unexamined Patent Publication (Kokai) No.
  • the method (2), straight barrel type snorkel method, unlike the method (1) has no significant limitation on equipment, and an example of this method is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 61-37912.
  • the method disclosed in this publication is shown in Fig. 35.
  • a molten steel 71 contained in a ladle 70 is sucked through a snorkel 72 into a vacuum tank 73.
  • An inert gas is blown into the molten steel within the snorkel 72 under the plane of projection of the snorkel 72 within the ladle 70, and, at the same time, an oxidizing gas is blown through a top lance 74 onto the surface of the molten steel within the vacuum tank 73.
  • the inner diameter of the snorkel 72 is determined so that the ratio of the inner diameter (D 1 ) of the snorkel 72 to the inner diameter (D 0 ) of the ladle 70, that is, D 1 /D 0 , is 0.4 to 0.8.
  • the depth of blowing of the inert gas is determined so that the ratio of the depth (H 1 ) of blowing of the inert gas as measured from the surface of the molten steel to the depth (H 0 ) of the molten steel within the ladle 70, that is, H 1 /H 0 , is 0.5 to 1.0.
  • the above method for vacuum refining of molten steel aims to efficiently carry out decarburization without the deposition of the metal, slag and the like within the tank.
  • Japanese Unexamined Patent Publication (Kokai) No. 2-133510 proposes a vacuum treatment apparatus comprising: a ladle for placing therein a molten metal; a vacuum tank having a snorkel, submerged in the molten metal, provided at the lower end of the vacuum tank; an evacuation pipe connected to a vacuum source for evacuating the interior of the vacuum tank; and a shield disposed in the interior of the vacuum tank, wherein the shield is kept at a height of 2 to m above the molten steel surface within the snorkel.
  • RH-OB is widely known as a method for oxygen blowing decarburization refining in vacuo.
  • this method for example, in the finishing of stainless steel, aluminum is added to the molten steel before the oxygen blowing decarburization and combustion is carried out using top-blown oxygen to raise the temperature of the molten steel (aluminum temperature elevation or temperature elevation by aluminum).
  • top-blown oxygen to raise the temperature of the molten steel
  • the depth of a cavity, of the molten steel, formed by a blown oxygen jet becomes large, leading to a fear of bricks at the bottom of the tank being damaged by the blown oxygen jet, which makes it difficult to conduct temperature elevation by aluminum under a high degree of vacuum.
  • the straight barrel snorkel type vacuum refining method is disadvantageous in that, as can be seen in the process for producing an ultra low carbon high chromium steel disclosed in Japanese Unexamined Patent Publication (Kokai) No. 57-43924, there is a limitation on the decarburization in a degassing period due to the difficulty of maintaining the agitating force and, as can be seen in the vacuum refining method disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2-305917, an attempt to improve the reduction rate in the degassing period results in remarkable wear of refractories.
  • introduction of aluminum as a reducing agent into the molten steel within the vacuum tank in order to recover a metal by reduction of a metal oxide, for example, chromium oxide, causes a rise in temperature of the molten steel by heat generated by thermit reaction, or scattering (bumping) of the molten steel or slag by a reduction reaction involving instantaneous evolution of CO gas, resulting in melt loss of refractories within the tank and deposition of the metal or slag, which is an obstacle to the operation.
  • a metal oxide for example, chromium oxide
  • US-A-4,152,140 discloses a DH vacuum degassing equipment in which a molten steel is sucked into the lower end of the vacuum tank.
  • JP-A-02-77517 discloses a method and a device for heating a RH vacuum degassing vessel.
  • JP-61-37912 discloses a vacuum refining apparatus in which a molten steel is sucked into the lower end of the vacuum tank.
  • a general object of the present invention is to solve the above problems created in oxygen blowing decarburization of a molten steel by the above-described RH-OB, VOD, or a refining method using a vacuum refining apparatus comprising a vacuum tank having a one-legged, straight barrel snorkel.
  • a more specific object of the present invention is to provide a method for vacuum decarburization refining of a molten steel that, even when the concentration of carbon in the molten steel is in a high concentration region, can inhibit the deposition of a splash onto the inner wall of the vacuum tank, the nozzle submerged in the molten steel, and the top-blown lance, prevent loss of a metal in the molten steel, for example, loss of chromium by oxidation, and, at the same time, reduce the fixation between the snorkel arid the ladle by the slag.
  • Another object of the present invention is to provide means that does not increase flow resistance of an exhaust gas in a passage, shields the upper part of the vacuum tank and the oxygen lance from radiated heat during the vacuum decarburization refining, inhibits the entry of dust created by splashing of the molten steel into an evacuation system, and at the same time prevents clogging of the evacuation system with the dust.
  • a still another object of the present invention is to provide means that, during oxygen blowing decarburization in a high carbon concentration region, can prevent a metal oxide formed during the oxygen blowing decarburization from flowing out into the outside of the tank.
  • a further object of the present invention is to provide a method for adding aluminum that, at the time of raising the temperature using aluminum, can prevent the production of a metal oxide other than Al 2 O 3 and the deposition of a large amount of the metal.
  • a still further object of the present invention is to provide a degassing method that can efficiently produce an ultra low carbon steel while preventing the production of a metal oxide in the molten steel.
  • a refining method wherein a molten steel, which has been decarburized in a converter to regulate the carbon content to not more than 1% by weight (all "%" in the following description being by weight) is charged through a vacuum tank snorkel into a vacuum tank in a straight-barrel type vacuum refining apparatus; and in the vacuum tank, decarburization refining is carried out in such a manner that the carbon content of the molten steel is divided into a high carbon concentration region, which is a reaction region where the decarburization reaction rate is governed by the feed of an oxygen gas blown through a top-blown lance into the molten steel, and a low carbon concentration region which is a reaction region where the decarburization reaction rate is governed by movement of carbon in the molten steel, the degree of vacuum within the vacuum tank is regulated for each carbon concentration region and, at the same time, the flow rate of the oxygen gas blown through the top-blown lance is
  • the above refining method can enhance the oxygen efficiency in decarburization and at the same time can prevent the occurrence of splash within the snorkel and the fixation of slag in the nozzle submersed portion.
  • the degree of vacuum within the vacuum tank in the aluminum temperature elevation period is closely regulated according to the following conditions. This can prevent the deposition of the metal caused by splash or the oxidation of the metal.
  • Introduction of aluminum and quick lime in an amount of 0.8 to 4.0 times the amount (kg) of aluminum added in the aluminum temperature elevation period and, in addition, introduction of a slag component, such as quick lime, in the oxygen blowing decarburization period in a high carbon concentration region to maintain the slag thickness at 100 to 1000 mm are also effective in preventing splash and in accelerating the softening of slag.
  • the regulation of the depth of immersion of the snorkel in the molten steel in the aluminum temperature elevation period and the regulation of the immersion depth of the snorkel in the molten steel in the oxygen blowing decarburization period respectively to 200 to 400 mm and 500 to 700 mm can accelerate the reduction of a metal oxide (for example, Cr 2 O 3 in refining of stainless steel) by a reaction with carbon contained in the steel, permitting the oxygen efficiency in decarburization to be kept on a high level.
  • a metal oxide for example, Cr 2 O 3 in refining of stainless steel
  • degassing is carried out under reduced pressure.
  • an inert gas is injected from the low position of the ladle into the molten steel, of which the carbon concentration has been brought to around 0.01% by the oxygen blowing decarburization in such an atmosphere that the degree of vacuum within the snorkel is in the range of from 10 to 100 Torr, so as to bring K value, defined by the following equation, to the range of from 0.5 to 3.5, thereby agitating the molten steel.
  • K log ⁇ S ⁇ H v ⁇ Q/P ⁇
  • the degassing treatment can maintain the renewal of the interface at a activated surface, which is a substantial gas/metal reaction interface, enabling a high-purity molten steel having an attained carbon concentration of not more than 10 ppm to be effectively produced.
  • an inert gas for agitation is injected into the molten steel in the flow rate range of from 0.1 to 3.0 Nl/min/ton-steel (in terms of flow rate per ton of molten steel to be refined; hereinafter referred to as "Nl/min/t") in an atmosphere having a low degree of vacuum of not more than 400 Torr, or alternatively, it is possible to employ a method wherein, immediately after the degassing treatment, the pressure is returned to the atmospheric pressure, the vacuum tank is lifted, and, simultaneously with the lifting of the tank, aluminum for reduction is introduced into the molten steel and an inert gas for agitation is injected into the molten steel at a flow rate of 0.1 to 3.0 Nl/min/t during the introduction of aluminum for reduction and at a flow
  • the present invention provides a vacuum decarburization refining apparatus that can inhibit the deposition of splash (droplets) created by splashing or bumping, or dust formed by solidification of the splash onto the inner wall of the vacuum tank and the snorkel submerged in the molten steel, which is a major problem to be solved by the invention.
  • the vacuum decarburization refining apparatus has the following construction.
  • At least one burner is provided on the side wall, in an upper tank, in the vicinity of the canopy of the vacuum tank, and a space having a larger inner diameter than the inner diameter of the snorkel is provided in a lower tank in the vacuum tank.
  • a shielding section which has at its center a space having an inner diameter smaller than each tank and larger than the outer diameter of the top-blown lance, is provided, between the lower tank and the upper tank at a position which receives enough radiated heat to melt the deposited metal, integrally with the side wall of the vacuum tank.
  • the vacuum tank having the above construction permits the influence of a high temperature, around a hot spot created by the blowing of oxygen through the top-blown lance and the decarburization reaction, on the refractories in the side wall of the lower tank to be avoided, and at the same time enables the metal deposited on the shielding section to be melted by radiated heat. Further, dust constituted by splash which has ascended to the upper tank without being deposited onto the shielding section and has been deposited in the vicinity of the canopy, is melted by means of the burner, flows downward and is removed.
  • the evacuation duct disposed between the vacuum tank and a gas cooler for cooling an exhaust gas comprises an ascendingly inclined section inclined upward from an duct inlet provided in the upper tank of the vacuum tank and a descendingly inclined section inclined downward from the top of the ascendingly inclined section. Therefore, splash of the molten steel and dust, which, together with an exhaust gas, have entered the evacuation duct are collected in a dust pot provided below the descendably inclined section without being deposited within the evacuation duct.
  • a major object of the present invention is to increase the oxygen efficiency in decarburization while minimizing splash, bumping and other unfavorable phenomena created in the course of refining. Since, however, means is provided which, even when splashing or the like is created, can effectively avoid or remove droplets or dust derived from the splashing and the like, the degree of vacuum within the vacuum tank can be always kept on a desired level, realizing stable operation.
  • a vacuum decarburization refining system 10 comprises: a vacuum tank 15 comprising a cylindrical refractory; a ladle 13 containing a molten steel 11; and an evacuating apparatus 16 for evacuating the interior of the vacuum tank 15.
  • the vacuum tank 15 comprises a lower tank and an upper tank.
  • the lower tank constitutes a snorkel 14 submerged in the molten steel 11, while a top-blown lance 18 for blowing an oxygen gas into the molten steel 11 is liftably provided in the canopy of the upper tank.
  • the vacuum tank 15 is provided with a lift drive 17 for vertically moving the vacuum tank 15, and a nozzle (a porous plug) 19 for blowing an inert gas into the molten steel is provided at a low position of the ladle 13, for example, the bottom.
  • An oxygen gas flow rate control value 20 for regulating the flow rate of the oxygen gas blown through the top-blown lance 18 is disposed on the inlet side of the top-blown lance 18, and an inert gas flow rate control valve 21 for regulating the flow rate of the inert gas is provided on the inlet side of an inert gas suction nozzle.
  • These control valves for regulating the flow rates of the oxygen and inert gases are controlled by a controller 23 and the like.
  • a vacuum gage 22 for measuring the degree of vacuum within the vacuum tank 15 is provided at a predetermined position of the vacuum tank 15 or the evacuation system.
  • the vacuum decarburization refining system is constructed so that a signal corresponding to the degree of vacuum measured with the vacuum gage 22, a signal on the position of the snorkel 14 relative to the ladle 13, a signal indicating the concentration of carbon in the molten steel 11 and other signals are input into the controller 23 and, according to these input signals and an operating procedure described later, the controller 23 controls the evacuating apparatus 16 and the lift drive 17 so that the evacuating apparatus 16 and the lift drive 17 perform respective necessary operations.
  • the carbon concentration of the molten steel 11 may be directly measured, or alternatively may be determined by calculation based on the carbon concentration before the refining and the history of a change in concentration of a CO gas in the exhaust gas.
  • the ladle 13 is a nearly cylindrical vessel, for a molten steel, lined with a refractory such as an alumina-silica refractory.
  • decarburization refining of a molten steel is carried out under reduced pressure using the above apparatus.
  • a decarburization refining process as a finishing process of a stainless steel, wherein decarburization is carried out through aluminum temperature elevation-oxygen blowing decarburization-degassing-optional reduction with aluminum to bring a carbon concentration to a predetermined value, will be described by way of example.
  • the snorkel 14 provided at the lower part of the vacuum tank 15 is submerged, for example, in a molten stainless steel 11 having a chromium concentration of 16% and a carbon concentration of 0.7% within the ladle 13.
  • the interior of the vacuum tank 15 is evacuated by means of the evacuating apparatus 16 to maintain the degree of vacuum, P, within the vacuum tank on a predetermined level. This permits the molten steel 11 within the snorkel 14 to be sucked, causing the surface of the molten steel to ascend through the snorkel 14, which, as shown in Fig. 1, results in a change in depth h of immersion of the snorkel 14 and depth H of the molten steel within the ladle 13.
  • bringing the G value, defined by the following equation (1), to not more than -20 in an aluminum combustion period in an early stage (temperature elevation period) can inhibit excessive production of chromium oxide during the blowing of oxygen.
  • an important operating factor in the application of the equation (2) is the partial pressure of CO (P CO ) in an atmosphere represented by the degree of vacuum during operation, and the molten steel temperature (T) is a very important additional factor. Therefore, introduction in advance of aluminum or the like having higher affinity for oxygen than chromium and carbon followed by oxygen blowing to raise the molten steel temperature by utilizing the heat of oxidation is effective in inhibiting the oxidation of chromium in the oxygen blowing decarburization period.
  • the degree of vacuum in the aluminum temperature elevation period is kept on a high level as much as possible to burn only aluminum in this period.
  • the oxidation of chromium is prevented during the temperature elevation period by regulating the degree of vacuum within the tank so as to maintain the G value, defined by the equation (1), at a value of not more than -20.
  • the G value defined by the equation (1)
  • aluminum for temperature elevation is introduced in portions during temperature elevation/oxygen blowing, because introduction of aluminum all at once before the oxygen blowing followed by temperature elevation while oxygen blowing with aluminum dissolved in the molten steel creates such an unfavorable phenomena that aluminum in the molten steel within the vacuum tank is temporarily used up during the temperature elevation period and, in this state, even when the G value is brought to not more than -20, the oxidation of chromlum often occurs.
  • the distance between the surface of the molten steel sucked into the snorkel in the oxygen blowing period and the canopy of the vacuum tank, that is, the freeboard, is preferably not less than 6 m from the viewpoint of preventing spitting in the aluminum temperature elevation period and preventing splash, created in the subsequent decarburization refining period, from reaching the canopy.
  • temperature elevation period refers to a period between the initiation of oxygen blowing and the point of time when the oxygen blowing proceeds to the accumulated amount of oxygen represented by the following equation (3).
  • Amount of oxygen blown in temperature elevation period (Nm 3 ) Amount of aluminum added (kg) x purity of aluminum x 33.6/54
  • the G value is brought to the range of from -35 to -20.
  • the degree of vacuum is such that the G value exceeds -20, as indicated by a solid line in Fig. 2, the oxidation of chromium is promoted.
  • oxygen blowing decarburization under such a high vacuum that the G value is less than -35, as indicated by a dotted line in Fig. 2 leads to splashing, resulting in a remarkably deteriorated operation efficiency.
  • the G value in each of the above periods is regulated to a predetermined value as follows.
  • the degree of vacuum P is measured with the vacuum gage 22.
  • the temperature T of the molten steel is previously provided based on the temperature history for each carbon concentration predicted from the temperature before the treatment. Based on these data, the G value is determined in the controller 23 according to the equation (1).
  • the degree of vacuum B is regulated based on the results so that the G value falls within the above range.
  • quick lime (CaO) in an amount corresponding to 0.8W Al to 4.0W Al (kg), wherein W Al represents the amount of aluminum added at the time of the temperature elevation (kg), is introduced.
  • the resultant slag should be discharged into the outside of the tank before the degassing as a later step.
  • Al 2 O 3 produced by the aluminum temperature elevation as such, flows out into the outside of the tank, however, the slag floating in the ladle is solidified in an early stage because Al 2 O 3 per se is an oxide having a very high melting point. This makes it difficult to conduct sampling of the molten steel and, in addition, leads to a problem of fixing the snorkel to the ladle.
  • CaO is added in the above amount in the aluminum temperature elevation period to form a calcium aluminate compound (12CaO.7Al 2 O 3 ), a low-melting compound, improving the percentage liquid phase of the slag and consequently avoiding the above operation problems.
  • the depth of the snorkel submerged in the molten steel in the vacuum tank in the aluminum temperature elevation period is preferably in the range of from 200 to 400 mm, from the viewpoint of suitably bringing Al 2 O 3 and CaO produced by the oxygen blowing temperature elevation into contact with each other in the molten steel within the snorkel to accelerate the production of a calcium aluminate compound.
  • the immersion depth is less than 200 mm, as shown in Fig. 5, the time of contact between Al 2 O 3 and CaO in the molten steel within the snorkel is so short that Al 2 O 3 and CaO are discharged outside the system before the production of the calcium aluminate compound.
  • the G value is brought to the range of from -35 to -20 in a high carbon concentration region where the carbon concentration is the critical carbon concentration (0.1 to 0.3 wt%) or more, and, at the same time, the following requirements are satisfied.
  • the metal oxide is composed mainly of chromium oxide (Cr 2 O 3 ). Since Cr 2 O 3 is a high-melting oxide, the presence of Cr 2 O 3 results in a remarkably lower percentage of liquid phase of the slag.
  • the slag on the ladle becomes very rich in Cr 2 O 3 , and, even when the calcium aluminate is formed, the solidification of the slag on the surface of the molten steel within the ladle remarkably proceeds. This deteriorates the workability and, for example, makes it difficult to sample the molten steel.
  • maximizing the opportunity for contact of a metal oxide produced by oxygen blowing (in the present invention, the metal oxide will be hereinafter described as Cr 2 O 3 by taking oxygen blowing decarburization refining of a stainless steel as an example) with carbon contained in the molten steel within the snorkel of the vacuum tank to accelerate the reduction reaction within the snorkel is important from the viewpoint of preventing the oxidation loss of chromium in the oxygen blowing decarburization period and efficiently carrying out the oxygen blowing decarburization while maintaining the oxygen efficiency in decarburization on a high level.
  • One requirement for this according to the present invention is to form the activated surface, in the oxygen blowing decarburization period, in a proportion of not less than 10% of the total surface area of the molten steel and not less than 100% of the surface blown by the oxygen gas jet.
  • the carbon content of the molten steel to be decarburization-refined has been divided into a high carbon concentration region and a low carbon concentration region with the critical carbon concentration as a boundary between the high carbon concentration region and the low carbon concentration region, and, for each region, the optimal flow rate of an oxygen gas (oxygen blowing rate), the rate of a reduction in flow rate of the oxygen gas, the flow rate of an inert gas for agitation, the degree of vacuum in the vacuum tank, the immersion depth (immersion ratio) of the snorkel and the like have been investigated.
  • oxygen gas oxygen blowing rate
  • the rate of a reduction in flow rate of the oxygen gas the rate of an inert gas for agitation
  • the degree of vacuum in the vacuum tank the immersion depth (immersion ratio) of the snorkel and the like
  • the oxygen blowing decarburization refining reaction is generally divided into a high carbon concentration region, which is a reaction region where the decarburization rate (-d[C]/dt) is governed by the feed rate of the oxygen gas (a region governed by the feed of oxygen), and a low carbon concentration region which is a reaction region where the decarburization rate is governed by the moving speed of carbon in the molten steel (a region governed by the movement of carbon in the steel).
  • the critical carbon concentration ([%C]*) at which the region changes from the region governed by the feed of oxygen to the region governed by the movement of carbon in the steel is approximately in the range of from 0.1 to 0.3% by weight, although the critical carbon concentration somewhat varies depending upon the chromium content and the operating conditions.
  • the flow rate of the oxygen gas in the high carbon concentration region is limited to 3 to 25 Nm 3 /h/t.
  • the reason for this is as follows.
  • the flow rate of the oxygen gas in the high carbon concentration region is less than 3 Nm 3 /h/t, the decarburization rate of the molten steel is likely to fall, making it necessary to prolong the refining time, which lowers the productivity.
  • the upper limit of the flow rate of the inert gas for agitation is preferably 4.0 Nl/min/t.
  • the occurrence of splash in the high carbon concentration region becomes the most serious problem in stabilizing the operation.
  • the high carbon concentration region is the so-called "most active decarburization period.” During this period, the evolution of the CO gas is most active, which induces splashing. Therefore, in order to prevent splashing and to carry out oxygen blowing decarburization refining without causing significant deposition of the metal, the prevention of splashing in the high carbon concentration region is very important.
  • the present invention in the oxygen blowing decarburization period in the high carbon concentration region, quick lime or the like is added all at once or in portions to the tank, and oxygen blowing decarburization is carried out in such a state that slag having a thickness of 100 to 1000 mm in terms of the thickness in a stationary state is held on the surface of the molten steel within the snorkel.
  • Splashing created in the oxygen blowing decarburization is known to be created by rebounding of a top-blown jet and by bursting of CO gas bubbles, produced within the molten steel (bubble breaking) on the surface of the molten steel.
  • the attainable height of the splash is governed by the initial speed at the time of formation of splash (initial speed) and the Co gas evolution rate (that is, flow rate of the exhaust gas). Therefore, lowering the oxygen blowing speed per se is effective in reducing the attainable height of the splash.
  • the lowering in oxygen blowing speed leads directly to lowered throughput speed. Therefore, this means cannot be useful means from the viewpoint of maintaining the high productivity.
  • the reduction in initial speed immediately after the formation of splash is important from the viewpoint of reducing the attainable height and scattering distance of splash while maintaining the high productivity.
  • a suitable slag layer is formed on the surface of the molten steel.
  • the slag layer reduces the energy of the splash particles, thereby significantly relaxing the later scattering behavior.
  • the thickness of the slag layer to be held on the molten steel within the vacuum tank is preferably 100 to 1000 mm in terms of the thickness in a stationary state on the surface of the molten steel within the snorkel.
  • the thickness of the slag layer is less than 100 mm, the energy loss of the splash is small, making it impossible to relax the later scattering behavior.
  • the thickness exceeds 1000 mm the arrival of the top-blown oxygen jet onto the surface of the molten steel per se is inhibited, resulting in lowered oxygen efficiency in decarburization.
  • the composition of the slag to be accumulated on the surface of the molten steel can be provided by incorporating a slag material, such as quick lime, all at once or in portions into the vacuum tank in the high carbon concentration region, where splash particles are most actively produced in the oxygen blowing decarburization period and the carbon concentration is the critical carbon concentration or more.
  • This composition can protect the refractories constituting the snorkel and can prevent the solidification of the cover slag.
  • the oxygen blowing conditions according to the present invention are characterized by the rate of reduction in flow rate of the oxygen gas (oxygen blowing rate) in the low carbon concentration region.
  • oxygen blowing rate the rate of reduction in flow rate of the oxygen gas in the low carbon concentration region.
  • the reduction rate in this region has not been fully taken into consideration.
  • bringing the reduction rate to the range of from 0.5 to 12.5 Nm 3 /h/t/min has realized very effective operation.
  • the reduction rate of the flow rate of the oxygen gas in the low carbon concentration region is less than 0.5 Nm 3 /h/t/min, the reduction in evolution of CO gas is so small that the amount of splash created is excessive. Further, the amount of chromium oxidized attributable to excessive feed of the oxygen gas is increased.
  • the decarburization reaction in the low carbon concentration region is in a "region governed by the movement of carbon in the steel," the mass transfer of the carbon in the molten steel should be accelerated beyond the mass transfer in the high carbon concentration region in order to maintain the oxygen efficiency in decarburization at a high level.
  • the cover slag, within the snorkel, used for the prevention of splash in the high carbon concentration region should be discharged outside the tank as much as possible during the oxygen blowing decarburization period in the low carbon concentration region.
  • the flow rate of the inert gas for agitation is brought to a range of from 0.3 to 10 Nl/min/t, preferably 5 to 10 Nl/min/t, in the low carbon concentration region, and the immersion depth of the snorkel is increased and/or decreased in a predetermined range.
  • the immersion depth of the snorkel in the low carbon concentration region is decreased and/or increased in a predetermined range. This fluctuates the surface of the molten steel in the ladle and accelerates the heat transfer from the molten steel to the slag on the ladle, causing remelting of the slag, which facilitates sampling of the molten steel and, in addition, enables fixation between the snorkel and the ladle to be fully avoided.
  • the variation in the immersion depth of the snorkel may be semi-continuously carried out in a range of from 0.1 to 0.6 in terms of h/H wherein h represents the immersion depth of the snorkel and H represents the depth of the molten steel within the ladle.
  • the immersion depth of the snorkel is varied only by decreasing the immersion depth from the viewpoint of promoting the circulation of the molten steel and discharging the slag in an earlier stage.
  • the h/H value is less than 0.1, the discharge of the slag is significantly promoted. This, however, causes Cr 2 O 3 produced by oxygen blowing to be simultaneously discharged outside the tank before the reduction of Cr 2 O 3 with carbon contained in the molten steel, leading to increased chromium loss.
  • the decarburization refining is carried out in such a manner that an oxygen gas flow rate control valve 20, an inert gas flow rate control valve 21, a lift drive 17, and an evacuating apparatus 16 are controlled to maintain the oxygen gas flow rate (Q) at 3 to 25 Nm 3 /h/t, the inert gas flow rate (N) at 0.3 to 4.0 Nl/min/t, and the immersion ratio (h/H) at 0.1 to 0.6 as shown respectively in Figs. 11, 13, and 10 through the operation of the controller 23 or by the operations of an operator while monitoring or estimating a change in the concentration of carbon in the molten steel 11 within the snorkel 14 in the vacuum tank.
  • an oxygen gas flow rate control valve 20 an inert gas flow rate control valve 21, a lift drive 17, and an evacuating apparatus 16 are controlled to maintain the oxygen gas flow rate (Q) at 3 to 25 Nm 3 /h/t, the inert gas flow rate (N) at 0.3 to 4.0 Nl/min/t, and the immersion ratio (h/
  • the decarburization refining is continued in such a manner that, as shown in Figs. 10 to 14, the oxygen gas flow rate (Q) is reduced at a reduction rate (R) of 0.5 to 12.5 Nm 3 /h/t/min by regulating the oxygen gas flow rate control valve 20 and, in addition, as shown in Fig. 16, the immersion depth (h) of the snorkel in the molten steel 11 is reduced in a predetermined range by operating the lift drive 17.
  • the reduction rate of the oxygen gas flow rate (Q) is the magnitude of the slope of the oxygen gas flow rate (Q) over the time, that is, the derivative time of the oxygen gas flow rate (Q), and is expressed in Nm 3 /h/t/min.
  • the oxygen gas flow rate (Q), the inert gas flow rate (N), the degree of vacuum (P) (regulation based on G value), the immersion ratio (h/H), the immersion depth (h) of the snorkel in the molten steel 11, the thickness of the slag having a regulated composition and the like are regulated to respective predetermined values, thereby simultaneously satisfying the following objects (i) to (iii).
  • the molten steel which has been subjected to oxygen blowing decarburization in this way, is then degassed under a high degree of vacuum.
  • maintaining the concentration of oxygen in the steel on a high level during degassing is effective in efficiently accelerating the decarburization reaction in the degassing period.
  • spontaneous evolution of a CO gas from the interior of the molten steel is known to be a major decarburization reaction site.
  • maintaining the concentration of oxygen in the steel on a high level is useful particularly in an early stage of the degassing.
  • This method is based on the optimization of the concentration of oxygen in the steel specified by the equilibrium condition of the partial pressure of CO (P CO ) represented by the carbon concentration and the degree of vacuum within the tank and makes it possible to maintain the degassing rate on a high level during degassing.
  • the solubility of carbon in the steel based on the equilibrium condition specified in this case is unsatisfactory, even though the carbon concentration during the stop of oxygen blowing is in the range of from 0.01 to 0.1%.
  • the amount of oxygen to be consumed by the degassing reaction is insufficient, disadvantageously making it difficult to produce a high-purity steel by the melt process.
  • the degree of vacuum within the tank is lower than 100 Torr, chromium is excessively oxidized in the last stage of the oxygen blowing period.
  • the attained degree of vacuum at the time of degassing should be as high as not less than 5 Torr.
  • the degree of vacuum is low and less than 5 Torr, it is difficult to ensure a satisfactory driving force in the production of a high-purity steel by the melt process, disadvantageously resulting in lowered degassing rate.
  • oxygen is reblown (reblowing) in an amount of 0.3 to 5 Nm 3 per ton of the molten steel preferably for about 2 to 3 min, and, in addition, the flow rate of the gas for agitation during the degassing is regulated to the range of 2.5 to 8.5 Nl/min/t while bringing the amount of the slag 12-1 within the tank during the stop of oxygen blowing to not more than 1.2 tons/m 2 per unit sectional area of the steel bath portion in the vacuum tank.
  • Reblowing of oxygen is carried out from the viewpoint of increasing the concentration of oxygen in the steel in order to further accelerate the internal decarburization.
  • the degree of vacuum is most preferably in the range of from 5 to 30 Torr. In this case, when the degree of vacuum is excessively high and exceeds 5 Torr, the dissolution of oxygen in the molten steel based on the equilibrium condition becomes difficult. On the other hand, when oxygen is reblown under a low degree of vacuum of less than 30 Torr, the blown oxygen is consumed by the oxidation of chromium rather than enrichment of oxygen in the molten steel.
  • the amount of oxygen blown at that time is preferably in the range of from 0.3 to 5 Nm 3 per ton of the molten steel.
  • the amount of oxygen reblown is less than 0.3 Nm 3 /t, oxygen to be consumed in the degassing is not satisfactorily enriched, even though the degree of vacuum within the tank at the time of reblowing is in the proper range.
  • oxygen is reblown in an amount exceeding 5 Nm 3 /t, the oxygen enrichment effect is saturated. In this case, on the contrary, there is a fear of oxygen being consumed by the oxidation of chromium.
  • the reason why the flow rate of the gas for agitation is regulated in the range of 2.5 to 8.5 Nl/min/t is as follows.
  • the amount of circulated molten steel is unsatisfactory due to a shortage of agitating force, inhibiting the promotion of the internal decarburization, which disadvantageously lowers the degassing rate per se.
  • the gas flow rate exceeds 8.5 Nl/min/t, the circulation acceleration effect is saturated.
  • an attack on the refractory by the gas is intensified, unfavorably resulting in damage to the refractory.
  • the amount of the slag within the tank during the stop of oxygen blowing is brought to not more than 1.2 tons/m 2 per unit sectional area of the steel bath portion in the vacuum tank.
  • the amount of the residual slag within the tank exceeds 1.2 tons/m 2 per unit sectional area of the steel bath portion in the vacuum tank, the contact between the molten steel surface to be a reaction site in the decarburization reaction and the high vacuum atmosphere is blocked, resulting in a remarkably lowered area of effective reaction interface. This makes it difficult to maintain the degassing rate on a high level.
  • What is particularly important in ensuring the activated surface is to completely discharge chromium oxide and slag into the outside of the snorkel at the time of surface decarburization, because when chromium oxide or slag produced during the oxygen blowing decarburization is left even in a small amount on the activated surface, the surface decarburization is inhibited, leading to a lowering in decarburization rate.
  • an inert gas should be injected from the low portion of the ladle which is distant by Hv from the molten steel surface within the snorkel (still molten steel surface), imparting a predetermined agitation intensity K to the activated surface.
  • K log ⁇ S ⁇ Hv ⁇ Q/P ⁇ wherein P represents the degree of vacuum, Torr; S represents the gas bubble activated area, m 2 ; Q represents the flow rate of an inert gas blown, Nl/min/t; and Hv represents the distance from the molten steel surface within the snorkel to the position where the inert gas is blown, m.
  • metal for reduction is further introduced to reduce a metal oxide (for example, Cr 2 O 3 ) produced during the oxygen blowing, followed by recovery of the metal.
  • a metal oxide for example, Cr 2 O 3
  • the oxidation of chromium contained in the molten steel that is, the production of Cr 2 O 3
  • a reducing agent should be added to recover the chromium component.
  • silicon a ferrosilicon alloy
  • aluminum should be used as the reducing agent.
  • the equilibrium carbon concentration in the equation (7) is greatly influenced by the equilibrium partial pressure of CO, that is, the degree of vacuum in operation.
  • the reaction represented by the equation (7) proceeds more significantly with an increase in the degree of vacuum.
  • the present invention provides a technique that solid aluminum, immediately after the introduction of aluminum, is brought into contact with solid slag to allow the thermit reaction to proceed moderately to form molten slag which covers the molten steel to inhibit the pick-up of nitrogen.
  • the flow rate of the argon gas for agitation during the introduction of aluminum for reduction is brought to the range of from 0.1 to 3 Nl/min/t, and the degree of vacuum is brought to a low value of not more than 400 Torr. Thereafter, the pressure is returned to the atmospheric pressure, and the tank is lifted. At the same time, the flow rate of the argon gas for agitation is brought to the range of from 5 to 10 Nl/min/t.
  • Maintaining the flow rate of the argon gas for agitation in the proper range during the introduction of aluminum for reduction and, at the same time, bringing the degree of vacuum to a low degree of vacuum of not more than 400 Torr permits the agitation force within the vacuum tank to be suitably maintained and can inhibit the suspension of the molten steel and the slag, inhibiting excessive progress of the thermit reaction represented by the equation (6), which can inhibit an extreme increase in the temperature of the molten steel.
  • Suppression of the agitation during the introduction of aluminum for reduction can inhibit the dissolution of aluminum in the molten steel and permits a direct reaction of aluminum with the slag to improve the reduction rate of Cr 2 O 3 .
  • the flow rate of the argon gas for agitation in the aluminum introduction period is brought to the range of from 0.1 to 3 Nl/min/t.
  • the argon gas flow rate in this period exceeds 3 Nl/min/t, the thermit reaction represented by the equation (6) excessively proceeds and, at the same time, the emulsion of the slag and the metal is also intensified, making it difficult to prevent bumping.
  • the argon gas flow rate is less than 0.1 Nl/min/t, the introduced aluminum is deposited within the vacuum tank, often making it impossible to properly introduce aluminum, or otherwise creating the penetration of the molten steel into a porous plug provided at the bottom of the ladle. This raises an operation problem that, when the flow rate is increased in the subsequent stage, a desired flow rate cannot be ensured.
  • the agitating force becomes excessive. Specifically, the effective contact area between the slag and the metal is increased, and, in addition, the equilibrium partial pressure of CO, having a close relationship with the degree of vacuum at that time, is lowered. This shifts the reaction equilibrium in the equation (7) towards the right side, instantaneously causing significant acceleration of the reaction involving the evolution of the CO gas. This makes it difficult to prevent bumping.
  • a preferred method for solving this problem is that, immediately after degassing, the pressure is returned to the atmospheric pressure, the vacuum tank is lifted, and aluminum is then introduced. Direct contact of aluminum, for reduction, with the slag deposited onto the upper part of the wall of the ladle improves the reduction efficiency of Cr 2 O 3 . Further, as described above, when a large amount of Cr 2 O 3 is produced during the oxygen blowing, the amount of slag within the vacuum tank inevitably becomes large. In this case, the slag on the upper part of the ladle after the lifting of the vacuum tank heaps into a mound.
  • a snorkel 14 of a straight-barrel type vacuum tank is submerged in a molten steel 11 having a chromium concentration of not less than 5% contained in a ladle 13.
  • the interior of the snorkel 14 is evacuated.
  • an argon gas as an inert gas for agitation is fed through a porous plug 19 provided at the bottom of the ladle 13 into the molten steel while blowing an oxygen gas onto the molten steel from above the molten steel within the vacuum tank, thereby carrying out oxygen blowing decarburization refining in vacuo. After the oxygen blowing is stopped, degassing is carried out under a high degree of vacuum.
  • Fig. 19 (B) the pressure of the interior of the snorkel 14 is returned to the atmospheric pressure, and the snorkel 14 is pulled up. At the same time, the flow rate of the argon gas for agitation is increased to the range of from 5 to 10 Nl/min/t.
  • numeral 12-1 designates melted slag
  • numeral 12-3 solid slag present outside the vacuum tank.
  • the pressure within the snorkel 14 is returned to the atmospheric pressure (Fig. 20 (A)), and, in addition, as shown in Fig. 20 (B), the snorkel 14 is pulled up.
  • aluminum 26 for reduction is simultaneously introduced.
  • the flow rate of the argon gas for agitation is regulated in the range of from 0.1 to 3 Nl/min/t during the introduction of aluminum for reduction.
  • the flow rate of the argon gas for agitation is increased to the range of from 5 to 10 Nl/min/t to fluctuate the molten steel as shown in Fig. 20 (C), thereby promoting the contact of the solid or deposited slag with the high-temperature molten steel.
  • the relationship between the recovery of Cr 2 O 3 and the flow rate of the argon gas for agitation in this embodiment is shown in Fig. 22.
  • the flow rate of the argon gas for agitation is 5 to 10 Nl/min/t, the recovery of Cr 2 O 3 can be improved and, in addition, an increase in pick-up of nitrogen can be prevented.
  • a snorkel in a lower tank of the vacuum tank is submerged in the molten steel within the ladle.
  • the fluidity of the molten steel such as molten stainless steel
  • high-temperature refining such as oxygen blowing decarburization
  • the wear of the refractories constituting the snorkel leads to a lowering in rate of operation of the vacuum refining apparatus, and the lowered throughput capacity in the vacuum refining makes it impossible to treat the object steel species. As a result, the production of high grade steels per se becomes difficult.
  • the wear of the snorkel used in the vacuum refining in an early stage leads to increased cost of refractories constituting the snorkel, and a lot of time and labor are required in the replacement of the vacuum tank and the snorkel.
  • the above problem has been solved by immersing the snorkel, after the completion of the refining, in slag having a regulated composition to coat the slag onto the surface of the snorkel.
  • the slag after the completion of the refining under reduced pressure is regulated so as to comprise 55 to 90% by weight in total of Al 2 O 3 and CaO, 1 to 10% by weight of Cr 2 O 3 , and 7 to 25% by weight of SiO 2 with the balance consisting of 2 to 10% by weight of at least one member selected from FeO, Fe 2 O 3 , and MgO.
  • the slag coating on the snorkel has poor corrosion resistance and, in this case, the effect of protecting the snorkel cannot be attained by the slag coating.
  • the total amount of Al 2 O 3 and CaO exceeds 90% by weight, the melting point of the slag becomes high and slagging is poor. This makes it difficult to coat the slag onto the snorkel and is an obstacle to the reduction of the chromium oxide in the reduction refining as the previous step.
  • the slag composition formed upon completion of the reduction refining is less than 7% by weight, the slag has lowered viscosity and higher melting point. In this case, as with the case of increased total amount of Al 2 O 3 and CaO, the slagging is poor, and coating becomes difficult.
  • the slag composition FeO, Fe 2 O 3 , and MgO as the balance are produced in the refining under reduced pressure and included in the previous step, and the slag contains 2 to 10% by weight of at least one member selected from FeO, Fe 2 O 3 , and MgO.
  • the amount of FeO, Fe 2 O 3 , and MgO is increased, the corrosion resistance of the slag is lowered due to a lowering in melting point.
  • the amount of MgO is less than 2% by weight, the melt loss of refractories constituting the snorkel is significant, while when the amount exceeds 10% by weight, MgO should be additionally added.
  • SiO 2 comprises a slag component (the content of SiO 2 in the slag included: 30% by weight) included at the time of tapping of the molten steel 11 from a decarburization refining furnace (not shown), such as a converter, into the ladle 13, and Si (0.03 to 0.20% by weight) contained in the molten steel 11 before the decarburization refining under reduced pressure.
  • a decarburization refining furnace not shown
  • Si (0.03 to 0.20% by weight
  • the SiO 2 content can be previously determined by analysis.
  • the whole amount of Si in the molten steel 11 is expressed in terms of SiO 2 , and the total of both the SiO 2 contents is regarded as the SiO 2 content.
  • the SiO 2 content in terms of the total of both the above contents is regulated in the range of from 7 to 25% by weight by regulating any one of or both the amount of the inflow slag and the amount of silicon added to the molten steel 11.
  • the amount of CaO to be added in the degassing refining is determined from the amount of chromium oxide and the like to be reduced in the reduction refining by the following method.
  • the amount of chromium oxide produced is predicted from the above-described decarburization refining conditions, that is, the amount of blown oxygen and the attained final carbon concentration.
  • a method may be used wherein the molten steel or slag is analyzed, and the amount of metallic aluminum to be added for reducing the amount of the produced chromium oxide and, in addition, the amount of Al 2 O 3 produced are determined according to the equation (8) : Cr 2 O 3 + 2 Al ⁇ Al 2 O 3 + 2 Cr
  • the amount of CaO is determined from the amount of Al 2 O 3 , and regulation is carried out so that the total amount of CaO and Al 2 O 3 is 55 to 90% by weight.
  • the regulation of CaO and Al 2 O 3 may be made by varying the amount of both or any one of CaO and Al 2 O 3 added.
  • the amount of Cr 2 O 3 is determined by the amount of metallic aluminum added in the reduction refining, and decreases with increasing the amount of the metallic aluminum added. Therefore, the amount of Cr 2 O 3 is regulated in the range of from 1 to 10% by weight.
  • FeO, Fe 2 O 3 , and MgO as the balance are produced in the refining under reduced pressure and included in the previous step.
  • the amount of slag included the amount of metallic aluminum added in the reduction refining and the like are regulated so that the slag contains 2 to 10% by weight of at least one member selected from FeO, Fe 2 O 3 , and MgO.
  • the Al 2 O 3 /CaO ratio in the slag is brought to the range of from 0.25 to 3.0.
  • Regulated slag 12 after each refining and refining under reduced pressure is melted at a temperature of 1650 to 1750°C.
  • the pressure of the interior of the vacuum tank 15 and the snorkel 14 are returned to the atmospheric pressure.
  • the snorkel 14, the pressure of which has been returned to the atmospheric pressure is lifted above the slag 12 and then stands by.
  • both the temperature of chromia-magnesia bricks 28 constituting the inside of the snorkel 14 and the temperature of high alumina, prepared unshaped refractories 29 constituting the outside of the snorkel 14 are substantially the same as the temperature of the slag 12, that is, 1650 to 1750°C.
  • the temperature is lowered to 1200 to 1300°C by the standing-by of the snorkel 14 in the lifted state for about 0.5 to 1 min.
  • the snorkel is submerged in the slag 12 layer by 270 to 530 mm from the front end of the snorkel 14, and, immediately after that, the snorkel 14 is slowly lifted to form a 30 mm-thick coating 32.
  • the snorkel 14 is further allowed to stand by for additional 5 min.
  • the temperature of the surface of the coating 32 has reached about 800°C
  • the snorkel 14 is submerged in the molten steel 11 within the next ladle 13, followed by the next refining under reduced pressure. Thereafter, the formation of the coating 32 on the snorkel 14 and the refining under reduced pressure are repeatedly carried out.
  • the snorkel may be again submerged in the slag 12 and allowed to stand by, thereby forming a 60 mm-thick coating.
  • the coating 32 formed by double coating procedure has the effect of preventing both breaking and melt loss of refractories derived from spalling created by a rapid temperature change from 1750°C to the atmospheric temperature, or from 800°C to the temperature of the molten steel 11 of around 1750°C at the time of immersion of the snorkel in the molten steel.
  • the bricks 28, 29 constituting the snorkel 14 are held by a core metal 27 provided with a flange 31, and the prepared unshaped refractory brick 29 is held by a stud 30.
  • the apparatus of the present invention is characterized by means that, when dust and the like are created, can trap and melt the dust in the vacuum tank and, also when a dust-containing gas is introduced into an evacuation duct, can inhibit the deposition and accumulation of the dust, and, in addition, can prevent damage to refractories constituting the lower tank in the vacuum tank caused by heat of radiation from the molten steel (mainly from a hot spot) during the vacuum refining.
  • a vacuum decarburization refining apparatus according to one embodiment of the present invention will be described.
  • a vacuum decarburization refining apparatus 10 comprises: a ladle 13 that is provided, at the bottom thereof, with an inert gas blowing nozzle 19 and contains a molten steel 11; a vacuum tank 15 provided with a snorkel 14, submerged in the molten steel 11 within the ladle 13, and an evacuation hole 16-1 connected to an evacuation apparatus (not shown); and an oxygen lance 18 that is liftably provided in a canopy 35 of the vacuum tank 15.
  • the ladle 13 is a substantially cylindrical iron container, and the inner wall in contact with the molten steel 11 is lined with a refractory, for example, an alumina-silica or alumina-zircon refractory.
  • a refractory for example, an alumina-silica or alumina-zircon refractory.
  • the molten steel 11 within the ladle 13 is agitated by an ascending, and the kinetic energy of, an inert gas blowing into the molten steel 11 through a gas blown nozzle 19 provided in the ladle 13, thereby enhancing the vacuum refining reaction in the molten steel 11.
  • the vacuum tank 15 is a container for vacuum refining that is mainly lined with a refractory brick such as 4 magnesia-chromia brick (a part of the container may be constituted by a prepared unshaped refractory).
  • the vacuum tank 15 comprises an upper tank 33 and a lower tank 34, the lower end of the lower tank serves as a snorkel 14 and is submerged in the molten steel.
  • the molten steel ascends through the snorkel, permitting a molten steel surface 11-1 different from the molten steel surface within the ladle 13 to be formed within the snorkel.
  • An oxygen gas is blown against the surface through the lance.
  • the snorkel refers to a lower end portion of the vacuum tank which is located below the position, of the vacuum tank, where the uppermost surface of the sucked molten steel is in contact with the vacuum tank.
  • the snorkel 14 is in a substantially cylindrical form having an inner diameter D F , and the snorkel 14, particularly on its portion which is submerged in the molten steel 11 and through which the molten steel ascends, is coated with a prepared unshaped refractory, for example, an alumina-silica, by casting.
  • a prepared unshaped refractory for example, an alumina-silica
  • the present invention is characterized by providing a larger-diameter section 36, having an inner diameter D L larger than the inner diameter D F of the snorkel and having a length A in the vertical direction, in the lower tank 34 continued to the snorkel 14.
  • the larger-diameter section serves to disperse splash created by an oxygen gas jet blown through the oxygen lance 18 against the molten steel surface 11-1 and, at the same time, to reduce the thermal influence of a hot spot created by the oxygen gas jet or heat of radiation from the molten steel surface 11-1 on the side wall section of the vacuum tank, and is a constituent element important to the vacuum tank of the present invention.
  • the inner diameter D L of the larger-diameter section is specified, in relation with the position of a gas blown hole of the oxygen lance 18, so that the ratio of the inner diameter D L to the oxygen gas blowing distance L (distance between the lower end of the oxygen lance and the molten steel surface 11-1), D L /L, is in the range of from 0.5 to 1.2. This offers the above effect.
  • a smaller-diameter section (a diameter-reduced section) 37 having an inner diameter Ds is provided, at a position a vertical length A from the lower end of the larger-diameter section 36, connected to the larger-diameter section 36.
  • the smaller-diameter section 37 functions to inhibit the introduction of splash or dust into the upper tank in the vacuum tank, and melts dust and the like, deposited on the bottom face thereof, by heat of radiation from the molten steel surface to remove the dust and the like from the smaller-diameter section.
  • the ratio S S /S L is specified to the range of from 0.5 to 0.9.
  • the smaller-diameter section is provided at a position against which a stream of the oxygen gas blown through the lance does not directly impact and where melt loss of the refractory derived from the heat of radiation from the hot spot and the molten steel surface does not occur and only the dust deposited onto the refractory can be remelted (for example, at a position where the surface temperature of the refractory constituting the smaller-diameter section is 1200 to 1700°C).
  • the length A is specified to be 1 to 3 m.
  • the difference between the inner diameter D S of the smaller-diameter section and the outer diameter of the oxygen lance 18 in the radial direction is preferably small.
  • the difference d is preferably in the range of from 100 to 300 mm.
  • the melt loss of the refractory in the side wall section of the vacuum tank (freeboard section) not directly submerged in the molten steel 11 is governed by the surface temperature of the refractory, the temperature of the atmosphere gas, and the flow rate of a gas that collides with the working face of the refractory.
  • the hot spot In the impinging region (the hot spot) in which a jet stream of the oxygen gas blown from the oxygen lance 18 impinges with the molten steel 11, carbon contained in the molten steel is oxidized with the oxygen gas to evolve a CO gas and the temperature in the vicinity of the hot spot is as high as about 2400°C due to the calorific value involved in the decarburization reaction.
  • the CO gas flow rate also becomes maximum at the portion just above the hot spot immediately after the evolution of the CO gas.
  • the freeboard section in the vacuum decarburization refining undergoes wearing action due to heat of radiation, a gas stream or the like which occurs by the hot spot having a high-temperature and the portion just above the hot spot. Therefore, it is important to properly maintain geometrical arrangement between the hot spot and the freeboard section.
  • setting of the geometrical arrangement between the hot spot and the refractory of the vacuum tank in the above manner can minimize the melt loss of the refractory in the freeboard section, the oxygen lance and the like and, at the same time, can prevent the introduction of dust created by splashing of the molten steel 11 into the evacuation system, realizing the operation of vacuum decarburization refining with high productivity.
  • a vacuum decarburization refining furnace 10 according to the second preferred embodiment is substantially the same as that according to the first preferred embodiment, except that the structure of the smaller-diameter section 37 of the vacuum tank 15 in the vacuum decarburization refining apparatus 10 described in the first preferred embodiment has been changed to the structure of fan-shaped shields 38, 39, 40. Therefore, like parts have the same index numerals, and detailed description thereof will be omitted.
  • the fan-shaped shields 38-40 are provided so as to be different from one another in position as well as in level in the vertical direction. Further, as shown in Fig. 29, the shields are provided at a fan angle ⁇ for covering the whole molten steel surface within the vacuum tank except for the sectional area Ss in the space As defined by the shields.
  • the fan-shaped shield 38 is provided by fixing a core metal 41, with a cooling air passage 43 provided therein, onto the inner side of an iron skin 15-1 in the vacuum tank and fixing a prepared unshaped refractory, such as alumina castable refractory, onto the core metal 37 through a Y-shaped stud 42 mounted on the core metal 41.
  • a prepared unshaped refractory such as alumina castable refractory
  • provision, as the smaller-diameter section, of a plurality of fan-shaped shields so as to be different from one another in level can effectively shield the heat of radiation from the hot spot on the molten steel surface 11-1, and splash and in addition, enables vacuum decarburization refining while maintaining the evacuation passage in the vacuum tank 15 so as to avoid an increase in evacuation resistance.
  • the formation of the fan-shaped shield using a prepared unshaped refractory has been described. It is also possible to form the fan-shaped shield using a shaped refractory, for example, a magnesia-chromia refractory brick.
  • each fan-shaped shield may not be necessarily identical so far as the whole molten steel surface except for the space around the oxygen lance is covered with the surface of the fan-shaped shields. Further, the number of fan-shaped shields is not limited to three.
  • Figs. 27 and 28 show such a state that blowing is carried out under a low degree of vacuum within the vacuum tank. Therefore, in this state, the height of the surface of the molten steel within the snorkel is low.
  • a space is provided in the smaller-diameter section so that the oxygen nozzle 18 is passed through the space. Therefore, there is a possibility that an exhaust gas containing dust ascends through the space, reaches the side wall of the upper tank in the vacuum tank, particularly the canopy and the side wall near the canopy, causing the dust to deposit and accumulate.
  • the present invention further provides means for preventing the deposition of the dust.
  • burners 44-1, 44-2 are provided so that the front end thereof is located below the canopy 35 by a distance F (burner front end distance F).
  • these burners are inserted and provided in the upper tank 33 so as to face each other so that the gas ejection direction has a predetermined burner ejection angle ⁇ h to the vertical direction and a burner whirling angle ⁇ r.
  • the burner front end distance F is preferably in a range of from 0.3 to 3 m
  • the burner ejection angle ⁇ h is preferably in a range of from 20° to 90°
  • the whirling angle ⁇ r is preferably in a range of from 15° to 30°.
  • an oxygen gas, a fuel gas, or a mixed gas composed of the oxygen gas and the fuel gas blown through the burners 44-1, 44-2 into the upper tank 33 forms a whirling stream within the upper tank 33, permitting a refining gas evolved in the course of the oxygen blowing refining to be efficiently mixed with the oxygen gas, fuel gas and the like and, at the same time, permitting the temperature of the canopy 35 to be properly held.
  • the above burners are applied during the oxygen blowing decarburization refining, the surface temperature of the canopy is detected with a plurality of thermocouples buried in the canopy 35, and the surface temperature of the canopy is kept in a range of 1200 to 1700°C as shown in Fig. 31.
  • an inspection hole for measurement of the temperature may be provided in the side wall of the upper tank so that the surface temperature of the canopy is directly measured with an optical pyrometer.
  • the dust which has reached around the canopy, is melted and removed, preventing a lowering in yield of chromium or iron derived from the deposition of the dust.
  • the blowing of the oxygen gas through the oxygen lance 18 is ended, and an argon gas is injected from the low portion of the ladle 13 into the molten steel 11 to agitate the molten steel 11 in the snorkel 14.
  • the evacuating apparatus is stopped, the pressure within the snorkel 14 is returned to the atmospheric pressure, and the lower end of the snorkel 14 is pulled up from the molten steel 11 in the ladle 13 and is held in a standing-by state.
  • the surface temperature of the canopy is regulated in a predetermined temperature range (1200 to 1700°C) using the burners 44-1, 44-2.
  • the degree of vacuum within the vacuum tank is maintained at a predetermined value while sucking an exhaust gas evolved during the refining through a steam ejector.
  • the sucked exhaust gas is cooled by means of a gas cooler and fed into an exhaust gas treatment system.
  • the present invention further provides a vacuum refining apparatus that can prevent clogging of an evacuation duct with dust introduced into the evacuation duct, permitting the attained degree of vacuum within the vacuum tank to be maintained on a predetermined level and, in addition, can facilitate the removal of dust.
  • an evacuation duct 16-1 is provided in the upper tank of the vacuum tank 15, and an duct inlet 45 is connected to an inlet of a gas cooler 55 for cooling the exhaust gas through the duct.
  • a dust pot 53 for collecting the dust contained in the exhaust gas is provided in the course of the passage of the evacuation duct 16-1 having an actual length L 0 of about 15 to 50 m, and the evacuation duct extending from the upper tank to the dust pot is constructed so that the dust is not accumulated within the evacuation duct.
  • the evacuation duct 16-1 leading to the dust pot 53 comprises an ascendingly inclined section 46, having a total length of about 1.5 m, inclined upward from the duct inlet 45 at an inclination angle ( ⁇ 0 ) of 30° to 60°, and a descendingly inclined section 48, having a total length of about 1.5 m, inclined downward from the top 47 of the ascendingly inclined section 46 at an inclination angle of about 45°.
  • the adoption of an inclination angle exceeding 60° is difficult from the viewpoint of design due to the restriction of the system. Further, when the inclination angle exceeds 60°, the effect of dropping the dust on the ascendingly inclined section into the vacuum tank is substantially saturated. For this,reason, the upper limit of the inclination angle is 60°.
  • the actual length L 0 of the evacuation duct refers to the length of the evacuation duct along the evacuation direction, that is, the total length from the duct inlet to the gas cooler.
  • a heating device 49 is provided aslant toward the ascendingly inclined section 46 around the top 47 of the ascendingly inclined section 46 so that dust and the like accumulated on the top 47, the ascendingly inclined section 46, or the descendingly inclined section 48 are heat-melted and flow down into the vacuum tank 15 or the dust pot 53.
  • a branched section 50 is provided below the descendingly inclined section 48, and the dust pot 53 is detachably disposed at the lower part of the branched section 50 so that the dust and the like dropped along the inside of the inclined duct in the descendingly inclined section 48 are collected in the dust pot 53.
  • the evacuation duct 16-1 is constructed so that the flow direction of the exhaust gas is changed by about 90° in the branched section 50. Changing the direction and speed of the exhaust gas in this way can accelerate the dropping of the dust contained in the exhaust gas into the dust pot 53.
  • the body of the evacuation duct 16-1 further extends, from the end portion of the descendingly inclined section 48 as the branched section 50 located just above the dust pot 53, through a curved portion and a linear portion to an inlet of the gas cooler 55.
  • the system is constructed so that the actual length (L 0 ) of the evacuation duct 16-1 extending from the duct inlet 45 to the inlet of the gas cooler 55 and the inclination angle ( ⁇ 0 ) are if necessary set as desired.
  • the gas cooler 55 is a cooling device, for an exhaust gas, with a cooling plate or the like provided therein, and is constructed so that the gas within the cooler is discharged by means of an evacuation apparatus (not shown). Solid particles (dust) in the exhaust gas, which have collided against the cooling plate or the inner wall of the cooler and consequently lost speed, are collected in a inverted conical lower part of the gas cooler 55 and hence may be recovered according to need.
  • a pot detaching device 52 comprises: a guide rod 58 having in its front end a cotter hole 57; a hydraulic cylinder 60 for vertically moving the guide rod 58 through a disc spring 59; an upper flange 63 for fixing the hydraulic cylinder 60; and a fixed flange 61 for movably holding the guide rod 58 through a guide hole (not shown) for connection to a receiving flange 62 of the dust pot 53.
  • the dust pot 53 is a substantially cylindrical container, having a bottom section, made of steel or a casting and comprises: a receiving flange 62 disposed in the upper end portion; a guide rod insertion hole for inserting therein the guide rod 58 of the pot detaching device 52 provided in the receiving flange; and a pair of suspension trunnions 54 provided, so as to face each other, in the outer periphery of the dust pot 53.
  • the dust pot 53 is constructed so that, if necessary, the inner wall may be covered with a refractory lining material, such as a castable refractory lining material.
  • the dust pot 53 When a large amount of dust has been collected in the dust pot 53, the dust pot 53 may be detached using the pot detaching device 52, permitting the dust collected in the dust pot 53 to be easily removed and, at the same time, enabling maintenance, such as cleaning around the branched section 50, to be carried out.
  • the dust pot 53 may be detached from the evacuation duct 16-1 as follows. At the outset, a chain 65 is mounted on a metal hanger 64 mounted on the trunnion 54 of the dust pot 53, and the dust pot 53 is supported by means of a chain block (not shown). In this state, fixing bolts and nuts between the receiving flange 62 and the fixing flange 61 are removed.
  • the hydraulic cylinder 60 is operated using a hydraulic unit (not shown) to depress the guide rod 58 while pressing the disc spring 59.
  • the cotter 56 is removed from the cotter hole 57, and, in addition, the dust pot 53 is lowered using the chain block.
  • the guide rod 58 may be pulled out from the guide rod inserting hole 62-1 of the receiving flange 62 to completely separate the dust pot 53 from the evacuation duct 16-1, followed by removal of the dust, containing metal and the like, collected in the dust pot 53.
  • the evacuation duct of the present invention can effectively prevent dust from accumulating within the duct. Therefore, a predetermined degree of vacuum can be maintained without increasing the pressure loss involved in evacuation of the evacuation duct.
  • the apparatus of the present invention has at least one of the above features, realizing stable operation of the vacuum refining apparatus.
  • vacuum oxygen blowing refining of a stainless steel according to one embodiment of the present invention was carried out using a vacuum oxygen blowing refining apparatus on a scale of 150 tons.
  • a molten steel having [%C] 0.6 to 0.7% and [%Cr] 10 to 20% was prepared by the melt process, and temperature elevation and oxygen blowing decarburization were carried out using an oxygen blowing refining apparatus shown in Fig. 1.
  • the oxygen blowing rate was regulated in such a manner that, for all the cases independently of the temperature elevation period and the decarburization refining period, the oxygen blowing rate was kept at a constant rate of 23.3 Nm 3 /h/t until [%C] reached 0.3%; when [%C] was in the range of from 0.15% to 0.05%, the oxygen blowing rate was reduced from 23.3 Nm 3 /h/t to 10.5 Nm 3 /h/t at a constant rate; and when [%C] reached 0.05%, the blowing of oxygen was stopped.
  • the flow rate of an argon gas for agitation was evenly 4.0 Nl/min/t for the temperature elevation period and 2.7 Nl/min/t for the decarburization refining period.
  • Run Nos. 1 to 5 fall within the scope of the present invention, and run Nos. 6 to 11 are comparative runs.
  • Run No. 7 is a run where, although the G value in the aluminum temperature elevation period was not more than -20 on the average, it exceeded -20 (maximum value -18) during the temperature elevation period. In this run, the oxidation of chromium proceeded in the period where the G value exceeded -20.
  • run No. 8 where the average G value (-18) in the decarburization refining period exceeded -20, the oxidation of chromium excessively proceeded.
  • run No. 9 is a run where although the average G value (-24) was in the range of from -20 to -35, it exceeded -20 in a part of the decarburization refining period. In this run, the oxidation of chromium proceeded during this period.
  • run No. 10 where the G value (-37) was less than -35 in a part of the decarburization refining period, splashing was significantly created in this period posing a problem of deteriorated operation, although the oxidation of chromium was prevented.
  • run No. 11 where aluminum for an increase in temperature was introduced at once during the temperature elevation/oxygen blowing period, the oxidation of chromium was increased in the temperature elevation period.
  • the G value in the decarburization refining period was regulated as specified in Table 1 (2).
  • decarburization refining was carried out in such a manner that in the course of the decarburization wherein [%C] of the molten steel was decreased from 0.7% to 0.05% (at the time of stopping of the oxygen blowing), [%Cr] and T were determined, and, based on the data, P within the vacuum tank was regulated to regulate the G value as shown in Table 1 (2).
  • the refining as indicated in Table 1 (2), good decarburization results could be obtained when the regulation was carried out so that, for the G value, the maximum value was -21 with the minimum value being -25 and the average value being -23.
  • Example 1 In order to demonstrate the effect attained by adding CaO, the procedure of Example 1 was repeated, except that CaO together with aluminum was introduced during the aluminum temperature elevation period.
  • Runs according to the present invention are shown in Tables 2 and 3.
  • Run Nos. 1 to 12 are runs according to the present invention.
  • Run No. 13 since the W CaO /W Al ratio was less than 0.8, the production of calcium aluminate was not accelerated, causing slag to remain solidified, which made it difficult to sample the molten steel and at the same time resulted in deteriorated oxygen efficiency in decarburization.
  • run No. 14 due to excessive CaO, the amount of slag was so large that the decarburization by oxygen jet in the decarburization period was inhibited. Run Nos.
  • Run Nos. 21 and 22 are runs where the flow rate of the argon gas for agitation in the decarburization period , was less than 1.7 Nl/min/t and exceeded 6.0 Nl/min/tl respectively.
  • oxygen blowing decarburization was carried out at an oxygen blowing rate of 24.0 Nm 3 /h/t until [%C] reached 0.05%. Further, for all the runs, the flow rate of an argon gas for agitation in the oxygen blowing decarburization period was 3.3 Nl/min/t.
  • Figs. 15 to 17 are graphs respectively showing the relationship between the oxygen efficiency in decarburization and the immersion ratio (h/H), the relationship between the oxygen efficiency in decarburization and the flow rate (N) of an inert gas and the relationship between oxygen efficiency in decarburization and the reduction rate (R) of the flow rate of an oxygen gas.
  • the oxygen efficiency in decarburization can be brought to not less than 65% by maintaining the immersion ratio (h/H) at 0.1 to 0.6 and maintaining the flow rate (N) of the inert gas at 0.3 to 4.0 Nl/min/t.
  • the oxygen efficiency in decarburization can be maintained at not less than 65% without deteriorating the productivity by bringing the reduction rate (R) of the oxygen gas flow rate to the range of 0.6 to 12.5 Nm 3 /h/t/min.
  • the hatched portion is a region where the productivity is deteriorated due to prolonged treatment time and the like in the whole refining treatment.
  • run No. 1 of Example 4 is a run where in the high carbon concentration region, the oxygen gas flow rate was maintained in the specified range, that is, at 3 to 25 Nm 3 /h/t, while, as specified in Table 5, maintaining the immersion ratio (h/H) and the inert gas flow rate (N) respectively at 0.3 and 1.7 Nl/min/t, and, in the subsequent low carbon concentration region, the oxygen gas flow rate (Q) was reduced at a rate of 6.7 Nm 3 /h/t/min and the immersion depth (h) of the snorkel 14 was decreased and/or increased.
  • the oxygen gas flow rate was maintained in the specified range, that is, at 3 to 25 Nm 3 /h/t, while, as specified in Table 5, maintaining the immersion ratio (h/H) and the inert gas flow rate (N) respectively at 0.3 and 1.7 Nl/min/t, and, in the subsequent low carbon concentration region, the oxygen gas flow rate (Q) was reduced at a rate of 6.7 Nm 3 /h/t/min and the immersion
  • Tables 7 and 8 show comparative runs No. 1 to No. 8 where the conditions were outside the scope of the present invention. For all of runs No. 1 to No. 8, the overall evaluation was poor (X).
  • Run No. 1 is a comparative run wherein the immersion ratio (h/H) was set at 0.06 which was a value outside the range (0.1 to 0.6) specified in the present invention.
  • the oxygen efficiency in decarburization in the high carbon concentration region was 43%, i.e., a lower value than the reference value 65% for the evaluation.
  • Run No. 2 is a comparative run wherein the oxygen gas flow rate (Q) was set at a value which was outside and higher than the range (3 to 25 Nm 3 /h/t) specified in the present invention. In this run, the oxygen efficiency in decarburization in the high carbon concentration region was as low as 45%.
  • Run No. 3 is a comparative run wherein the inert gas flow rate (N) was set at 0.15 Nl/min/t, i.e., a value outside the range (0.3 to 4.0 Nl/min/t) specified in the present invention. In this run, the oxygen efficiency in decarburization in the high carbon concentration region was 38%, a lower value than that in run No. 2.
  • Run No. 4 is a comparative run wherein the oxygen gas flow rate in the high carbon concentration region was set at a value which was outside and lower than the range (3 to 25 Nm 3 /h/t) specified in the present invention. In this run, the oxygen efficiency in decarburization in the high carbon concentration region was 42%, i.e., poor.
  • Run No. 5 is a comparative run wherein the reduction rate (R) of the oxygen gas flow rate in the low carbon concentration region was set at 0.2 Nm 3 /h/t/min, a value outside the range (0.5 to 12.5 Nm 3 /h/t/min) specified in the present invention. In this run, the oxygen efficiency in decarburization in the low carbon concentration region was as low as 31%.
  • Run No. 6 is a comparative run wherein the reduction rate (R) of the oxygen gas flow rate in'the low carbon concentration region was set at 16.2 Nm 3 /h/t/min, a value exceeding the range (0.5 to 12.5 Nm 3 /h/t/min) specified in the present invention. in this run, the amount of chromium loss or the like became large and not negligible, resulting in remarkably lowered productivity.
  • Run No. 7 is a last comparative run wherein the decarburization refining was carried out with the immersion depth (h) of the snorkel 14 submerged in the molten steel in the low carbon concentration region being fixed.
  • slag 12 was deposited on the molten steel surface at the inner wall of the ladle 13 and the outer wall of the snorkel 14, causing fixation between the ladle and the snorkel, which was an obstacle to the production.
  • the target carbon concentration of the steel species in all runs according to the present invention is not more than 0.002% (20 ppm).
  • the oxygen gas blowing rate during the oxygen blowing decarburization refining was kept constant, i.e., at 20 Nm 3 /h/t.
  • Run No. 15 is a comparative run wherein [%C] during a stop of oxygen blowing was 0.012% (lower than 0.02%). This resulted in increased oxidation of chromium during oxygen blowing.
  • Run No. 16 is a comparative run wherein [%C] during a stop in oxygen blowing was 0.125% (larger than 0.1%). This resulted in increased attained carbon concentration, making it impossible to produce desired stainless steel within a predetermined treatment time range.
  • Run No. 17 is a comparative run wherein the degree of vacuum during a stop of oxygen blowing was higher than the degree of vacuum specified in the present invention. In this run, due to an insufficient amount of oxygen during degassing, the decarburization could not be smoothly carried out.
  • Run No. 18 is a comparative run wherein the degree of vacuum during a stop of oxygen blowing was lower than the degree of vacuum specified in the present invention. In this run, the oxidation of chromium was unfavorably increased.
  • Run No. 19 is a comparative run wherein the attained degree of vacuum at the time of degassing was 12 Torr. In this run, the attained [%C] was high due to high equilibrium attained value.
  • Run No. 20 is a comparative run wherein the amount of oxygen reblown at the time of degassing was small. In this run, the amount of oxygen in the molten steel during degassing was so low that the decarburization could not smoothly proceed, resulting in high attained [%C].
  • Run No. 21 is a comparative run wherein the amount of oxygen reblown was large. In this run, chromium was oxidized due to the presence of excessive oxygen.
  • Run No. 22 is a comparative run wherein the degree of vacuum during reblowing of oxygen was higher than the range specified in the present invention. In this run, the amount of oxygen to be dissolved in the molten steel was insufficient. This caused a lowered decarburization rate, resulting in high attained [%C].
  • Run No. 23 is a comparative run wherein the degree of vacuum during reblowing of oxygen was lower than the range specified in the present invention. In this run, the oxidation of chromium proceeded.
  • Run No. 24 is a comparative run wherein the amount of an argon gas, which is one example of the gas for agitation, was smaller than that specified in the present invention.
  • Run No. 25 is a comparative run wherein the amount of the argon gas for agitation was larger than the range specified in the present invention. In this run, the attack of the refractory by the gas was severe, resulting in increased damage to the refractory.
  • Run No. 26 is a comparative run wherein the amount of the residual slag was increased. In this run, since the free surface, which is a main site for the decarburization reaction, was not satisfactorily ensured, the decarburization rate was so low that the attained [%C] was large.
  • This example was carried out using a vacuum degassing apparatus on a scale of 175 tons.
  • a molten steel having [%C] about 0.7% and [%Cr] not less than 5% (mainly 10 to 20%) was produced by the melt process in a converter
  • the oxygen blowing was stopped, the molten steel was degassed for 30 min by mere agitation through blowing of an inert gas from the bottom of the ladle, thereby bringing the C concentration to not more than 20 ppm.
  • Table 11 shows runs in the degassing period according to the present invention in comparison with comparative runs.
  • Run No. 5 is a comparative run wherein the K value exceeded 3.5.
  • the area of the gas bubble activated surface and the agitation intensity were satisfactorily maintained, and the attained [C] was low.
  • the erosion of the refractory was accelerated due to increased amount of the gas blown and the like. Therefore, conditions in run No. 5 are unsuitable for practical use.
  • the experiment in this example was carried out using a vacuum refining apparatus on a scale of 150 tons.
  • a molten crude stainless steel containing a chromium concentration of not less than 5% (mainly 10 to 20%) tapped from a converter was subjected to oxygen blowing decarburization refining in vacuum and then degassed, followed by addition of aluminum from the top of the vacuum tank to reduce Cr 2 O 3 produced during oxygen blowing, thereby recovering Cr.
  • the reduction time was 5 min.
  • Table 12 shows runs according to the present invention in comparison with comparative runs.
  • Runs No. 1 to No. 9 are runs according to the present invention.
  • Run No. 10 is a comparative run wherein the argon gas flow rate for agitation at the time of the introduction of aluminum for reduction was less than 0.1 Nl/min/t. In this run, the molten steel penetrated the porous plug, adversely influencing subsequent reduction.
  • Run No. 11 is a comparative run wherein the argon gas flow rate at the time of the introduction of aluminum was excessive. In this run, bumping occurred immediately after the introduction of aluminum.
  • Run No. 12 is a comparative run wherein the degree of vacuum during the reduction was higher than 400 Torr. In this run as well, bumping occurred. Run Nos.
  • 13 and 14 are comparative runs wherein the flow rate of the argon gas for agitation after the introduction of aluminum was less than 5 Nl/min/t or exceeded 10 Nl/min/t respectively. In this case, when the argon gas flow rate was less than 5 Nl/min/t, the recovery of Cr 2 O 3 was lowered. On the other hand, when the argon gas flow rate exceeded 10 Nl/min/t, a large pick-up of nitrogen was observed.
  • Run No. 15 is a comparative run wherein, when the deposition and solidification of Cr 2 O 3 -containing slag on the upper part of the wall of the ladle was observed, aluminum was introduced with the vacuum tank submerged in the molten steel. In this case, the recovery of Cr 2 O 3 was remarkably lowered.
  • a molten steel having a weight of 150 tons (t), comprising 13% by weight of chromium, 0.7% by weight of carbon, and 0.03 to 0.20% by weight of silicon was prepared by the melt process in a converter, and the molten steel was poured into a ladle 13.
  • the amount of slag poured from the converter was regulated to about 1000 kg (containing 30% by weight of SiO 2 ), and, in the vacuum refining apparatus 10 shown in Fig. 1, decarburization refining, degassing refining, and reduction refining were further carried out.
  • CaO and metallic aluminum were added in such a manner that CaO was dividedly added in two or three portions in the degassing refining and the metallic aluminum was dividedly added in two or three portions at the time of the initiation of the reduction of the reduction refining and in the course of the reduction refining.
  • the slag regulated to the composition shown in Table 13 was coated onto the snorkel 14 in its portion from the lower end thereof to 500 mm from the lower end to form a 30 mm-thick coating by single immersion. Further, the coating, standing-by and refining under reduced pressure were repeated. The results were compared with the conventional technique where there was no slag coating.
  • the present invention could increase the number of times of use of the snorkel by 1.5 times by virtue of a reduction in melt loss caused by the molten steel or slag and a reduction in spalling due to heat load.
  • the refractory cost of the snorkel of the present invention when the refractory cost of the conventional technique was presumed to be 1, was about 0.6, indicating that a marked reduction in cost of 40% could be achieved.
  • the slag for coating utilizes additives and the produced composition, which can effectively function also in decarburization refining and degassing refining in the refining apparatus under reduced pressure, particularly the acceleration of the reduction refining reaction, both the protection of the refractory constituting the snorkel and the acceleration of the refining can be synergistically utilized, simultaneously improving the refining efficiency, the service life of the snorkel, the reduction in refractory cost and the like.
  • Tables 14 and 15 show the results of vacuum decarburization refining for run Nos. 1 to 6 according to the present invention wherein vacuum decarburization refining conditions, such as the inner diameter D L and the inner sectional area S L (m 2 ) of a larger-diameter section 36 corresponding to a freeboard section, the length A of the larger-diameter section, the oxygen gas blowing distance L, and the inner sectional area S S (m 2 ) of a smaller-diameter section 37 having an inner diameter D S , were set at respective various values.
  • vacuum decarburization refining conditions such as the inner diameter D L and the inner sectional area S L (m 2 ) of a larger-diameter section 36 corresponding to a freeboard section, the length A of the larger-diameter section, the oxygen gas blowing distance L, and the inner sectional area S S (m 2 ) of a smaller-diameter section 37 having an inner diameter D S , were set at respective various values.
  • oxygen efficiency in decarburization refers to the proportion of the amount of the oxygen gas contributed to the decarburization reaction relative to the total amount of the oxygen gas fed through the oxygen lance. For runs No. 1 to No. 6 according to the present invention, the oxygen efficiency in decarburization was on a level of 68% to 78%.
  • the intimately mixing time is an index of the degree of agitation of the molten steel 11 in the vacuum refining and, for example, is expressed in the time taken from the introduction of a metallic element or the like as a label in the molten steel to the point of time when the concentration of the metallic element becomes even or constant.
  • the intimately mixing time was in the range of from 38 to 51 sec.
  • runs No. 1 to No. 4 are comparative runs wherein any one of the (D L /L) ratio and the (S S /S L ) ratio was outside the proper range.
  • Run No. 1 is a comparative run wherein the (D L /L) ratio was 0.4 and outside the proper range. In this run, the melt loss of the refractory corresponding to the horizontal position of the portion immediately above the molten steel surface was significant. As a result, run No. 1 was evaluated as unacceptable (X).
  • Run No. 2 is a comparative run wherein the (D L /L) ratio was 1.5, that is, significantly outside the proper range. In this run, the force by which oxygen was blown against the molten steel surface was so weak that the decarburization reaction efficiency was remarkably lowered. As a result, run No. 2 was evaluated as unacceptable (X).
  • Run No. 3 is a comparative run wherein the (S S /S L ) ratio was 0.4, that is, lower than the proper range. In this run, the flow resistance of the exhaust gas was so large that the degree of vacuum was lowered. As a result, run No. 3 was evaluated as unacceptable (X).
  • Run No. 4 is a comparative run wherein the (S S /S L ) ratio was 1.0, that is, larger than the proper range. In this run, the deposition of the metal within the vacuum tank was significant. As a result, run No. 4 was evaluated as unacceptable (X).
  • Runs No. 1 to No. 7 according to the present invention are runs wherein vacuum refining was carried out under down-blown oxygen decarburization refining conditions in vacuo as specified in Tables 17 and 18.
  • the results (deposition of metal, state of damage to refractory, and evaluation) are summarized in these tables.
  • the surface temperature in the canopy is the average temperature (°C) in each period, and, in the column of the burner blowing gas during oxygen blowing, the type of gas fed into burners 44-1 and 44-2 shown in Figs. 24 and 30 is indicated.
  • run No. 1 is a run according to the present invention wherein oxygen blowing decarburization refining was carried out in vacuo in such a manner that the front end distance L of the burner and the burner ejection angle ⁇ h were set respectively at 2.3 m and 50°, and the average surface temperature in the canopy in the oxygen blowing refining period, the average surface temperature in the canopy in the non-oxygen blowing refining period, and the average surface temperature in the canopy in the standing period were regulated respectively at 1520°C, 1500°C, and 800°C by means of the burners 44-1 and 44-2.
  • run No. 1 In run No. 1 according to the present invention, there was no deposition of the metal in the canopy 35, and the loss of the refractory was very small. As a result, run No. 1 was evaluated good (O).
  • Comparative runs No. 1 to No. 4 shown in Table 19 are comparative runs wherein the surface temperature of the canopy in any one of the oxygen blowing period (oxygen blowing refining period) and the non-oxygen blowing period (non-oxygen blowing refining period) was outside the predetermined range of from 1200 to 1700°C.
  • oxygen blowing refining period oxygen blowing refining period
  • non-oxygen blowing refining period non-oxygen blowing refining period
  • comparative run No. 1 is a comparative run wherein oxygen blowing decarburization refining was carried out in vacuo in such a manner that the front end distance L of the burner and the burner ejection angle ⁇ h were set respectively at 3.5 m and 65°, and the average surface temperature in the canopy in the oxygen blowing refining period, the average surface temperature in the canopy in the non-oxygen blowing refining period, and the average surface temperature in the canopy in the standing period were regulated respectively at 1150°C, 1100°C, and 800°C.
  • Runs No. 1 to No. 4 according to the present invention shown in Table 20 are runs wherein vacuum refining was carried out in such a manner that operation conditions, such as the inclination angle ( ⁇ 0 ) in an ascendably inclined section 46 of an evacuation duct 16-1 and the actual length (L 0 ) of the evacuation duct 16-1, were varied.
  • operation conditions such as the inclination angle ( ⁇ 0 ) in an ascendably inclined section 46 of an evacuation duct 16-1 and the actual length (L 0 ) of the evacuation duct 16-1.
  • run No. 1 according to the present invention in Table 20 is a run wherein vacuum refining was carried out for about 5 days in such a manner that the inclination angle ( ⁇ 0 ) was brought to 45°. the actual length (L 0 ) was brought to 22 m, and a dust pot 53 (metal pot) was disposed below a descendably inclined section 48.
  • comparative runs No. 1 and No. 2 in Table 21 are comparative runs wherein the inclination angle ( ⁇ 0 ) in the ascendably inclined section 46 was set at 15° for comparative run No. 1 and 0° for comparative run No. 2 which were outside the proper range of from 30 to 60°.
  • the deposition of dust in the duct inlet 45 was significant, the pressure loss in the evacuation duct 16-1 was increased, and the attained degree of vacuum was on a level of 35 Torr and 45 Torr respectively.
  • comparative runs No. 1 and No. 2 were evaluated as unacceptable (X).
  • Comparative run No. 3 is a comparative run wherein no metal pot was provided. In this run, the deposition of dust in the duct inlet 45 was very small. However, dust, which flowed in the duct beyond the top 47 of the ascendably inclined section 46, reached the gas cooler 55 without being collected. This caused remarkable damage to the gas cooler and resulted in a low attained degree of vacuum of 40 Torr.
  • Comparative run No. 4 is a comparative run wherein the actual length (L 0 ) of the evacuation duct 16-1 was 6 m, that is, outside the proper range (15 to 50 m). In this run, despite the provision of the metal pot 53, since the actual length (L 0 ) was short, the amount of inflow of the dust in the gas cooler 55 was increased, resulting in increased damage to the gas cooler 55.
  • optimal regulation of the pressure within a vacuum tank in an aluminum temperature elevation period and, in addition, feed of an oxygen gas at an optimal flow rate according to the carbon concentration while regulating the slag component in the oxygen blowing decarburization period can inhibit oxidation loss of chromium during the aluminum temperature elevation, can improve the oxygen efficiency in decarburization in the oxygen blowing decarburization period, and, in the high carbon concentration region, can prevent splashing within a snorkel of the vacuum tank and the fixation of the submerged section of the nozzle by slag. Therefore, the method for refining of a molten steel according to the present invention is very advantageous from the viewpoint of industry.

Claims (20)

  1. Verfahren zum Vakuumfrischen einer Stahlschmelze, die eine Kohlenstoffkonzentration von 1,0 bis 0,01 Gew.-% hat und in einer Pfanne enthalten ist, unter Verwendung einer Vakuumfrischvorrichtung mit einem Vakuumbehälter, der mit einem in die Stahlschmelze eingetauchten einsträngigen Geradrohrschnorchel versehen ist, wobei das Innere des Vakuumbehälters evakuiert wird, damit die Stahlschmelze durch das Innere des Schnorchels in den Vakuumbehälter aufsteigen kann, wobei das Verfahren die folgenden Schritte aufweist:
    eine Temperaturerhöhungsbehandlung durch Zugabe von Al zur Stahlschmelze im Vakuumbehälter und Einblasen von Sauerstoff unter einem vorbestimmten Vakuumgrad,
    ein Vakuum-Entkohlen/Frischen in einem hohen Kohlenstoffkonzentrationsbereich, wobei die Kohlenstoffkonzentration der Stahlschmelze nicht unter einer kritischen Kohlenstoffkonzentration im Bereich von 0,3 bis 0,1 Gew.-% liegt,
    ein Vakuum-Entkohlen/Frischen in einem niedrigeren Kohlenstoffkonzentrationsbereich als die kritische Kohlenstoffkonzentration, vorzugsweise bis ein Kohlenstoffgehalt im Bereich von 0,01 bis 0,1 Gew.-% liegt, und
    Beenden des Sauerstoffblasens durch eine Aufblaslanze und Durchführen einer Entgasungsbehandlung unter einem vorbestimmten Vakuumgrad,
    wobei im hohen Kohlenstoffkonzentrationsbereich ein Sauerstoffgas mit einem Durchfluß in einem Bereich von 3 bis 25 Nm3/h/Tonne Stahl in die Stahlschmelze durch die Aufblaslanze eingeblasen wird, die durch ein Einsatzloch in einem Dach des Vakuumbehälters anhebbar vorgesehen ist, und ein Inertgas mit einem Durchfluß in einem Bereich von 0,3 bis 10 Nl/min/Tonne Stahl von einer unteren Position der Pfanne eingespritzt wird und der Vakuumgrad im Behälter im hohen Kohlenstoffkonzentrationsbereich auf einen Wert in einem Bereich von -35 bis -20 ausgedrückt in GG, definiert durch die folgende Gleichung (1), reguliert wird, wodurch das Entkohlen/Frischen durch Sauerstoffblasen gefolgt von Entgasen durchgeführt wird: G = 5,96 x 10-3 x T x ln(P/Pco) wobei Pco = 760 x {10(-13800/T + 8.75)} x [%C]/[%Cr]    P < 760;
    wobei T eine Stahlschmelzentemperatur in K darstellt und
    P den Vakuumgrad im Behälter in Torr darstellt.
  2. Verfahren nach Anspruch 1, wobei der Durchfluß des von der unteren Position der Pfanne eingespritzten Inertgases im hohen Kohlenstoffkonzentrationsbereich oberhalb der kritischen Kohlenstoffkonzentration auf einen Bereich von 0,3 bis 4 Nl/min/Tonne Stahl gebracht wird und im niedrigen Kohlenstoffkonzentrationsbereich nicht oberhalb der kritischen Kohlenstoffkonzentration auf einen Bereich von mehr als 4 bis 10 Nl/min/Tonne Stahl gebracht wird.
  3. Verfahren nach Anspruch 1 oder 2, wobei in der Temperaturerhöhungsperiode infolge einer Oxidation von Aluminium in einem Schritt vor dem Entkohlen/Frischen durch Sauerstoffblasen die Temperatur der Stahlschmelze so erhöht wird, daß die Stahlschmelze in die Pfanne gegossen wird, der Schnorchel in die Stahlschmelze eingetaucht wird und zusätzlich der Vakuumgrad P in der Atmosphäre im Vakuumbehälter so gesteuert wird, daß sich ein durch die Gleichung (1) bestimmter G-Wert von höchstens -20 ergibt, Aluminium der Stahlschmelze im Vakuumbehälter mit dem gesteuerten Vakuumgrad zugegeben wird und das Sauerstoffgas durch die Aufblaslanze in den Vakuumbehälter geblasen wird, um Aluminium zu oxidieren, wodurch die Temperatur der Stahlschmelze erhöht wird.
  4. Verfahren nach einem der Ansprüche 1 bis 3, wobei Branntkalk in einer 0,8 bis 4,0 WAl (kg) entsprechenden Menge, wobei WAl die zur Temperaturerhöhung zugegebene Aluminiummenge darstellt, in den Behälter von der Temperaturerhöhungsperiode bis zur Periode der Entkohlung durch Sauerstoffblasen zugegeben wird und zusätzlich die Eintauchtiefe des Schnorchels in der Stahlschmelze in der Temperaturerhöhungsperiode im Bereich von 200 bis 400 mm liegt.
  5. Verfahren nach Anspruch 1 oder 2, wobei in der Entkohlungsperiode durch Sauerstoffblasen ein Inertgas in die Pfanne von der unteren Position der Pfanne unter Bedingungen eingespritzt wird, die eine Forderung erfüllen, daß eine aktivierte Oberflächengröße auf mindestens 10 % der gesamten Oberflächengröße der Stahlschmelze und mindestens 100 % einer durch einen Sauerstoffgasstrahl angeblasenen Oberfläche gebracht wird, wodurch die Stahlschmelze gerührt wird.
  6. Verfahren nach Anspruch 1, 2 oder 4, wobei im hohen Kohlenstoffkonzentrationsbereich in der Entkohlungsperiode durch Sauerstoffblasen Branntkalk oder dergleichen in den Vakuumbehälter auf einmal oder aufgeteilt eingeleitet wird, um Schlacke mit einer Dicke von 100 bis 1000 mm bezogen auf einen stillen Zustand auf der Oberfläche der Stahlschmelze im Schnorchel zu bilden, die anschließend beibehalten bleibt.
  7. Verfahren nach Anspruch 1 oder 2, wobei im hohen Kohlenstoffkonzentrationsbereich in der Entkohlungsperiode durch Sauerstoffblasen die Eintauchtiefe des Schnorchels in die Stahlschmelze im Bereich von 500 bis 700 mm liegt.
  8. Verfahren nach Anspruch 1, 2, 5 oder 7, wobei im niedrigen Kohlenstoffkonzentrationsbereich in der Entkohlungsperiode durch Sauerstoffblasen das Entkohlen durch Sauerstoffblasen durchgeführt wird, während der Sauerstoffgasdurchfluß in einem Bereich von 0,5 bis 12,5 Nm3/h/Tonne Stahl/min verringert wird und gleichzeitig die Eintauchtiefe h des Schnorchels in Beziehung zur Tiefe H der Stahlschmelze so reduziert wird, daß die Forderung h/H = 0,1 bis 0,6 erfüllt ist.
  9. Verfahren nach Anspruch 1 oder 3, wobei in der Entgasungsperiode die Entgasungsbehandlung so durchgeführt wird, daß beim Beenden des Sauerstoffblasens durch die Aufblaslanze der Vakuumgrad im Vakuumbehälter auf 10 bis 100 Torr gebracht und ein Inertgas vom unteren Abschnitt der Pfanne in die Pfanne eingespritzt wird, während die Menge der Schlacke im Schnorchel auf höchstens 1,2 Tonnen/m2 der geometrischen Querschnittfläche des Schnorchels reguliert und gleichzeitig der durch die folgende Gleichung (3) bestimmte K-Wert auf 0,5 bis 3,5 reguliert wird, wodurch die Stahlschmelze gerührt wird: K = log{S·Hv·Q/P} wobei
    K: Kennzahl einer Rührintensität an der aktivierten Oberfläche;
    S: aktivierte Oberflächengröße in m2;
    Hv: Tiefe des eingespritzten Inertgases in m;
    Q: Durchfluß des eingespritzten Inertgases in Nl/min/Tonne Stahl; und
    P: Vakuumgrad im Behälter in Torr.
  10. Verfahren nach Anspruch 1 oder 2, wobei beim Reduzieren eines Metalloxids mit Aluminium nach dem Abschluß des Entgasens in der Aluminiumreduktionsperiode Aluminium zur Reduktion in die Stahlschmelze zugegeben wird und in der Aluminiumzugabeperiode der Durchfluß eines Inertgases zum Rühren vom unteren Abschnitt der Pfanne in einen Bereich von 0,1 bis 3,0 Nl/min/Tonne Stahl gebracht wird, wobei der Vakuumgrad im Behälter auf höchstens 400 Torr gebracht wird, und nach dem Abschluß der Einleitung von Aluminium zur Reduktion der Vakuumgrad im Behälter zum atmosphärischen Druck zurückgeführt wird, gefolgt vom Anheben des Vakuumbehälters und Regulieren des Durchflusses des Inertgases zum Rühren in einem Bereich von 5 bis 10 Nl/min/Tonne Stahl, um das beim Sauerstoffblasen erzeugte Metalloxid zu reduzieren, wobei ein Metallelement zurückgewonnen werden kann.
  11. Verfahren nach Anspruch 1, wobei beim Reduzieren eines Metalloxids mit Aluminium nach dem Abschluß des Entgasens in einer Periode der Metalloxidreduktion durch Aluminium der Druck der Atmosphäre im Vakuumbehälter zum atmosphärischen Druck zurückgeführt, der Vakuumbehälter angehoben und gleichzeitig Aluminium zur Reduktion in die Stahlschmelze zugegeben wird und in der Aluminiumzugabeperiode der Durchfluß eines Inertgases zum Rühren in einen Bereich von 0,1 bis 3,0 Nl/min/Tonne Stahl gebracht wird und unmittelbar nach dem Abschluß der Zugabe von Aluminium zur Reduktion der Durchfluß des Inertgases zum Rühren in einen Bereich von 5 bis 10 Nl/min/Tonne Stahl gebracht wird, um das beim Sauerstoffblasen erzeugte Metalloxid zu reduzieren, und ein Metallelement zurückgewonnen wird.
  12. Verfahren nach Anspruch 1, wobei nach dem Abschluß des Entgasens oder der Reduktionsbehandlung mit Aluminium die Zusammensetzung von Schlacke nach Beendigung des Frischens so reguliert wird, daß die Schlacke gewichtsbezogen insgesamt 55 bis 90 % Al2O3 und CaO, höchstens 10 % Cr2O3 sowie 7 bis 25 % SiO2 aufweist, wobei der Rest aus insgesamt 2 bis 10 % mindestens eines Bestandteils besteht, der aus FeO, Fe2O3 und MgO ausgewählt ist, wobei das Al2O3/CaO-Verhältnis im Bereich von 0,25 bis 3,0 liegt, gefolgt von Auftragen der Schlacke auf die Oberfläche des Schnorchels der Frischvorrichtung nach dem Entkohlen/Frischen.
  13. Verfahren nach einem der Ansprüche 1 bis 12, wobei während oder nach Beendigung der Entkohlungs-/Frischperiode mit Sauerstoffblasen die Umgebung des Dachs mittels eines in den Vakuumbehälter eingesetzten Heizbrenners so erwärmt wird, daß die Oberflächentemperatur des Dachs im Vakuumbehälter auf 1200 bis 1700 °C gehalten wird.
  14. Vorrichtung zum Vakuum-Entkohlen/Frischen einer Stahlschmelze mit: einem einsträngigen Geradrohrschnorchel (14), der in eine Stahlschmelze (11) in einer Pfanne (13) eingetaucht wird; einem Vakuumbehälter (15), der am oberen Teil des Schnorchels (14) vorgesehen ist; einer Abgasevakuierungsvorrichtung zum Reduzieren eines Drucks im Vakuumbehälter (15) und einem Gaskühler (55) zum Kühlen eines aus dem Vakuumbehälter (15) abgegebenen Abgases; und einer Multifunktionslanze (18) mit der Funktion zum Blasen eines Sauerstoffgases auf die Oberfläche der Stahlschmelze im Schnorchel (14) und einer Funktion als Heizbrenner, dadurch gekennzeichnet, daß der Vakuumbehälter (15) einen oberen Behälter (33) und einen unteren Behälter (34) aufweist und ein Raumteilstück mit einem größeren Innendurchmesser (DL) als der Innendurchmesser (DF) des Schnorchels am unteren Ende des Vakuumbehälters vorgesehen ist.
  15. Vorrichtung nach Anspruch 14, wobei ein Teilstück (37) mit kleinerem Durchmesser mit einem Raumteilstück mit einem Innendurchmesser, der kleiner als der Schnorchel (14) und größer als der Außendurchmesser der Aufblaslanze (18) ist, zwischen dem unteren Behälter (34) und dem oberen Behälter (33) einstückig mit der Seitenwand des Vakuumbehälters (15) vorgesehen ist.
  16. Vorrichtung nach Anspruch 14 oder 15, wobei ein Heizbrenner (44) an der Seitenwand in der Umgebung des Dachs (35) des Vakuumbehälters (15) vorgesehen ist.
  17. Vorrichtung nach einem der Ansprüche 14 bis 16, wobei mindestens ein Heizbrenner (44) an der Seitenwand des oberen Behälters (33) so vorgesehen ist, daß eine Brenngasdüse des Brenners (44) 0,3 bis 3 m unterhalb der einen Teil des oberen Behälters (33) bildenden Oberfläche des Dachs (35) liegt und der Brenngasausstoßwinkel, definiert als Winkel der Richtung eines aus der Düse ausgestoßenen Brenngases zur senkrechten Richtung, in einem Bereich von 20° bis 90° liegt.
  18. Vorrichtung nach einem der Ansprüche 14 bis 17, wobei Heizbrenner (44) in einem Schwenkwinkel von 15° bis 30° so vorgesehen sind, daß sie zueinander weisen.
  19. Vorrichtung nach einem der Ansprüche 14 bis 18, wobei fächerförmige Schildteilstücke (38, 39, 40), die das Teilstück mit kleinerem Durchmesser in mehrere Teile aufteilen, einstückig mit der Seitenwand des unteren Behälters (34) in unterschiedlichen Höhen und so vorgesehen sind, daß sie das Raumteilstück des Schnorchels (14) mit Ausnahme des Raumteilstücks der Schildteilstücke abdecken.
  20. Vorrichtung nach einem der Ansprüche 14 bis 19, die ferner zwischen dem oberen Behälter (33) und dem Gaskühler (55) aufweist: ein aufsteigend geneigtes Teilstück (46), das von einem an der Seitenwand des oberen Behälters (33) vorgesehenen Kanaleinlaß (45) nach oben geneigt ist, ein absteigend geneigtes Teilstück (48), das vom Oberteil des aufsteigend geneigten Teilstücks (46) nach unten geneigt ist, und einen Staubsammeltopf (53), der am unteren Teil des absteigend geneigten Teilstücks (48) abnehmbar vorgesehen ist.
EP97913417A 1996-11-20 1997-11-20 Verfahren und vorrichtung zur vakuum-entkohlung/feinung von flüssigem stahl Expired - Lifetime EP0881304B1 (de)

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JP32617896A JPH10152721A (ja) 1996-11-20 1996-11-20 真空精錬装置
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JP33756596A JP3749582B2 (ja) 1996-12-02 1996-12-02 真空脱炭精錬炉
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JP34244296A JP3754154B2 (ja) 1996-12-07 1996-12-07 ステンレス鋼の真空下吹酸脱炭精錬方法
JP120301/97 1997-04-22
JP9120302A JPH10298635A (ja) 1997-04-22 1997-04-22 ステンレス溶鋼真空精錬炉用浸漬管の保護方法
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JP12318697 1997-04-24
JP9123186A JPH10298634A (ja) 1997-04-24 1997-04-24 ステンレス鋼の還元精錬方法
JP123186/97 1997-04-24
JP13429997 1997-05-07
JP13429997A JPH10310818A (ja) 1997-05-07 1997-05-07 ステンレス鋼の精錬方法
JP134299/97 1997-05-07
JP22064097 1997-07-31
JP22064097A JP3785257B2 (ja) 1997-07-31 1997-07-31 ステンレス鋼の脱ガス精錬方法
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